Mileage-Based Service Intervals
The recommended service interval mileage is usually every 12 thousand miles but would recommend every 8 to 10 thousand miles as a good distance to at least consider replacing the engine oil and filter as a minimum.
This obviously depends on usage and if used around town (such as short distance commuting or school runs) which can lead to fuel wash-down of the oil and this, in turn, can lead to lower performance and economy due to decreased cylinder compression. An easy way to check if this is occurring is to sniff the dipstick and if it smells of petrol then it is contaminated and needs replacing.
Time-Based Service Intervals
Another point to consider is that Service Intervals are not only based on mileage but also time periods (e.g. annually, two-years, etc) and should include fluids such as Coolant, Brake/Clutch Fluid and even Air Con Gas.
These fluids/gases can degrade over time and create problems within the systems. For example; Brake Fluid will absorb moisture from the atmosphere over time and this can lead to a reduced boiling point of the fluid which can then lead to loss of pressure within the system. Ideally, Brake Fluid should be replaced at least every two years regardless of mileage but can be checked for water content and boiling point readings. Also, consideration should be given as to where these vehicles originate from (e.g. hot, humid environment) and there is no guarantee that these fluids have been regularly replaced and may be long overdue.
Other components which can degrade over time rather than mileage are rubber components such as Tyres (which do have a date stamp on them and can invalidate insurance claims if excessively old and show signs of perishing), Drivebelts of which the rubber may have started perishing, wiper blades and many more components. These will need checking on an individual basis and a decision made as to when to replace.